The present invention relates generally to spacecraft, and more particularly, to a system that eliminates nulls created by summing signals derived from two broadbeam antennas on a spacecraft that point in different directions.
The telemetry, command and ranging (TC&R) antenna on many spacecraft is on the earth side of the spacecraft and a similar antenna is disposed on the opposite (anti-earth) side of the spacecraft, along the +Z and -Z axis. Typical spacecraft configurations apply the signal from both antennas to a single receiver through a coupler that normalizes signal levels from both antennas. The normal configuration involves the +Z antenna facing the earth so that the -Z antenna is completely masked by the spacecraft.
During orbit raising, and in a loss of lock scenario, the spacecraft can face in any direction. The incoming signal transitions from the +Z to the -Z antenna as the spacecraft rotates with respect to the earth. A problem occurs near the center (X/Y plane) where the spacecraft receives approximately equal signals from both antennas. The resulting signal has a "null" region. At the frequency of operation of the antennas, the wavelength is typically shorter than an inch. This means that the summing point for the +Z and -Z signals can add or subtract the signals depending on the exact orientation of the spacecraft. When two RF signals almost 180 degrees apart are added a strong signal is produced. However, when the signals subtract, the RF level is almost canceled (nulled out ) and the link to Earth is lost.
Previous approaches incorporate additional receivers with antennas having narrow beamwidths so that the signals can be combined at baseband. The baseband is sufficiently lower in frequency so that the nulling effect is not noticed. The prior approaches have also been less reliable as a result of assigning a different receiver to a signal shifted by 180 degrees.
Various approaches have been proposed to eliminate or ignore the antenna null. One approach involves constantly moving the spacecraft to assure that it will not be in a null situation for a significant period of time. This approach was determined to be undesirable since the same motion will put a good signal into a null. Another approach involves orienting the spacecraft so that the null will rarely face the earth, but this cannot be guaranteed during a loss of lock scenario.
Another solution involves switching the incoming signals between the two antennas. This solution sounds good until the reliability and failure modes are taken into consideration. A failure could lock the system on the wrong antenna and prohibit receiving a signal. This could be resolved by an additional receiver for each antenna which is quite expensive.
Therefore, it is an objective of the present invention to provide for a system that eliminates nulls created by two oppositely pointing antennas disposed on a spacecraft and thus eliminates loss of communication with and control of the spacecraft.